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Metal-matrix nano-composites

Metal-matrix composites (MMCs) have suffered from a couple of problems in motorsport. The first is that there have been restrictive regulations banning such materials, either explicitly or via a limit on specific modulus (the ratio of elastic modulus to density). The second barrier to their use in those series that have not already outlawed such materials is cost – below a certain level of budget, there is simply no need for a ban as the financial constraints on competitors means they can’t afford the expensive development that comes with them.

However, because motorsport’s use of MMCs has only ever been a very small niche compared to other markets such as automotive development of brakes, aerospace and defence and so on, the development of MMC materials has not been affected to any great extent by our enforced lack of interest.

Many researchers in the field of MMC development are concentrating on metal-matrix nano-composites, sometimes referred to as MMnC materials. The reinforcement in MMCs can take one of three forms – continuous fibres, whiskers/short fibres or particulates.

The clue to the thrust of MMnC development activities lies in the name. The reinforcement particles in nano-composite materials are very much smaller than has customarily been used, and the results of doing so seem promising. The particulate size is not necessarily in the realm of single-digit nanometres, but the scales are much smaller than the so-called micro-scale composites, and there is certainly some work being done where nano-scale reinforcements are below 10 nanometres in size. However, people working on nano-composites are generally discussing MMCs which have particulate reinforcement that can be several hundred nanometres in size rather than multiple micrometres.

We hear a great deal about the possible applications of carbon nanotubes and how they might bring about a revolution in materials, and there are a lot of researchers working on carbon nanotube-reinforced materials, particularly based on aluminium and titanium matrices. Such materials are often denoted by the letters CNT, so for example you will find a number of technical papers by searching for ‘Al-CNT’. CNT materials look extremely promising, but it remains to be seen whether the much-vaunted step change in materials properties actually happens.

However, much of the research into metal-matrix nano-composites is based on smaller-scale reinforcement using ‘traditional’ MMC fillers such as titanium diboride (TiB2), aluminium oxide (Al2O3) or silicon carbide (SiC), and these too are yielding improvements in mechanical properties such as strength and stiffness.  

There are some processing difficulties in dealing with very fine particulate reinforcements, such as the particulates not being dispersed evenly within the matrix and poor bonding between the matrix and reinforcement. Consequently there is a great deal of research being done on the processing of the materials. One reason that nano-composite materials aren’t more widely available is that the optimum processing methods and associated parameters have not yet been found. There are a number of liquid-state techniques, where the material is processed in its molten state, which show promise, but these often require some expensive secondary processing to achieve optimum properties.

Powder metallurgy methods are perhaps the closest to being perfected for commercial use. Here, the matrix and reinforcement are thoroughly mixed as finely divided powdered solids before being sintered under pressure, and there are commercially (though not widely) available aluminium nano-composites. Such materials have been used for engine components, and they offer distinct advantages for customers, not only in terms of improved properties but in the ease of manufacturing. Where micro-scale aluminium MMCs often required special cutting tools, aluminium nano-composites can be machined conventionally.

Written by Wayne Ward

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